1. Field of the Invention
The present invention pertains to three-dimensional imagery, and, more particularly, to a method and apparatus for enhancing resolution of three-dimensional imagery.
2. Description of the Related Art
A need of great importance in military and some civilian operations is the ability to quickly detect and identify objects, frequently referred to as “targets,” in a “field of view.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to be able to distinguish reliably between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe, and as a target or not.
Techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many areas, they are inherently limited by a number of factors. For instance, RADAR is limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such signals. Sonar suffers similar types of limitations. Thus, alternative technologies have been developed and deployed.
One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit radio waves and receive radio waves reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. Because of the short wavelengths associated with laser beam transmissions, LADAR data exhibits much greater spatial resolution than RADAR data.
LADAR systems are therefore useful in many applications for locating and identifying objects including, in military environments, automatic target recognition (“ATR”) systems. The resolution of data obtained from such a LADAR system is impacted by several design trade-offs including how many pixels are needed on target to provide the ATR system with enough information to autonomously identify targets. Other factors include the scan angles (which define the sensor field of view), the range, the range accuracy, and the range resolution of the system. The LADAR range is influenced by the laser power, the telescope collection aperture, and the detector response. The range accuracy is influenced by the sampling rate and convolution step size of the pulse capture electronics. The range resolution is influenced by the receiver bandwidth, laser pulse width, and the sampling rate of the pulse capture electronics.
A practical LADAR system design is based upon balancing several of these conflicting parameters. An ideal LADAR system would have high angular resolution, large scan angles (field of view), long range, a high range accuracy, and fine range resolution. The resulting LADAR system would be very expensive. High angular resolution implies that the angular spacing between pixels, i.e., reflected beamlets, is very small, which results in many more pixels on the target of interest making it easier to “see.” The larger the scan angles, the larger the area that can be searched for targets. The longer the range capability of the LADAR, the sooner the target can be found and the threat determined. Range accuracy is defined as how small of a range change can be resolved by the LADAR. Range resolution is defined as how close two laser returns can be spaced and still resolved. The cost of the system is also frequently a major driver in the design. Each of these parameters is traded against each other to get a system with acceptable performance characteristics for the particular application.
However, object identification requirements for three-dimensional sensors are becoming more demanding. This drives up the range accuracy, range resolution, and spatial resolution requirements for LADAR systems. This, in turn, drives up system costs by requiring higher tolerance components and application specific laser transmitters.
One alternative for enhancing the resolution of three-dimensional data is disclosed in U.S. Letters Pat. No. 5,898,483, entitled “Method for Increasing LADAR Resolution,” issued Apr. 27, 1999, to Lockheed Martin Corporation as assignee of the inventor Edward Max Flowers. The '483 patent discloses a technique wherein the LADAR data is generated from a split beam laser signal transmitted at a given elevation scan rate and a given azimuth scan rate, and the elevation scan rate by which the laser signal is transmitted is reduced by a first predetermined factor and azimuth scan rate by a second predetermined factor, wherein both of the factors are integers greater than 1. Although this technique mitigates some of the aforementioned problems, it requires increased hardware performance by the system. Furthermore, this technique only provides for a 2× spatial resolution increase and does not improve range accuracy or range resolution.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.